Note: Descriptions are shown in the official language in which they were submitted.
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SOLID MATRIX TUBE-TO-TUBE HEAT EXCHANGER
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of co-pending United States Patent
Application
Serial Number 12/625,237, filed November 24, 2009, the entirety of which is
incorporated by
reference herein.
BACKGROUND
1. Field of the Invention
The present invention relates to heat exchangers and in particular to the
transfer of
heat from one liquid, such as a slurry, to another.
2. The Prior Art
A heat exchanger is a device used to transfer heat from one medium to another.
In
industries such as the mining industry there are many processes that require
heating a mineral-
ore slurry. A slurry is a suspension of solid particles in a fluid. Slurries
contain solid particles
that have a tendency to settle. Some slurries also have a tendency to create
scale. Both of
these issues complicate performing heat-exchange processes, due to the need to
periodically
clean heat-transfer and other apparatus used in slurry processing
The high cost of energy makes heat exchangers crucial to the feasibility of
these
processes. Currently there are no heat exchangers on the market that meet this
need. As a
result, when it is necessary to heat a mineral-ore slurry in a countercurrent
manner (by cooling
another slurry passing in the opposite direction), very complex systems are
used, such as
contact heat exchangers in which steam is evolved from one slurry and absorbed
into the
other slurry in an adjacent manifold. This is the typical type of exchanger
used in Bayer
Process plants for producing alumina from Bauxite ore.
Slurries have been run through existing heat exchanger configurations such as
spiral
or plate heat exchangers. A spiral exchanger includes a pair of flat surfaces
that are coiled to
form two channels in a counter current arrangement with each channel having a
long curved
path. A plate exchanger is composed of multiple, thin, slightly separated
plates that have very
large surface areas and fluid passages for heat transfer.
Although spiral and plate exchangers are promoted as being able to handle
slurries,
they employ fluid passages having physical dimensions that are typically not
conducive to
maintaining a good suspension of solids in the slurry. Spiral and plate
exchangers do not have
easily accessible passages and in some cases have no access at all, leading to
high
maintenance costs. Spiral and plate exchangers can be used in some slurry
applications, but in
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fact they can be used only for relatively simple and dilute slurries in which
the slurry particles
stay easily suspended in the liquid.
Shell-and-tube exchangers are currently used in some slurry applications as
well. A
shell and tube exchanger consists of a series of tubes running through a shell
and containing a
medium to be either heated or cooled. The shell (or larger tube) contains a
second flowing
medium which either provides or absorbs the heat as required.
Currently it is possible to heat a slurry in the tube side of a shell-and-tube
exchanger
in which the shell contains a non slurry (liquid or steam), but it is not
possible to transfer heat
from a slurry to a slurry, because the slurry cannot be run in the shell side,
where large
particles will settle out, causing fouling and eventually blockage.
In the 1990's, several plants that processed nickel ore were installed in
Australia, all
using high temperature autoclaves. Extensive research was done for the design
of these plants
in order to select an effective heat exchanger. However, the best system which
was found was
a system in which steam was extracted by a slight vacuum from the slurry and
then
recondensed in the shell of a shell and tube exchanger to heat the slurry
passing in the
opposite direction. Although these Nickel plants involved a combined capital
investment of
over US $1 Billion, the designers were not able to find a better way to
transfer heat because
there existed no design for a simple countercurrent exchanger which could
transmit heat from
one slurry to another. These complex heat exchange systems recycle only about
70 percent of
the heat, representing a missed opportunity to significantly reduce operating
costs.
Graphite block heat exchangers are also known in the art. In these exchangers
graphite blocks are drilled with several closely spaced parallel holes for
carrying the solution
to be heated (or cooled), and other holes are drilled at right angles to them
to carry the heating
(or cooling) fluid. Such exchangers are widely used for heating and cooling
acids. However
they are limited in their usefulness for several reasons: graphite is soft and
cannot be used for
abrasive slurries; graphite can be oxidized and so is not chemically stable
for some
applications; graphite is brittle and has low strength, so the pressure at
which these
exchangers can operate is limited (high pressure causes cracks to form and
propagate from
tube to tube). Also, because of the brittleness of the graphite it is
difficult to establish a tube
header on the ends so that a simple straight-flowpath parallel tube
arrangement is not
possible. To avoid this problem, graphite exchangers are designed with a cross-
flow tube
arrangement but this is not nearly as effective as a parallel flow
arrangement.
Similar exchangers using other materials substituted for graphite have not
been
encountered. The main reason would seem to be that drilling or otherwise
machining blocks
formed from materials such as metal is expensive. An example of such an
exchanger is
disclosed in United States Patent No. 1,799,626, disclosing tubes cast in a
metal block. The
tubing would not be effective in accomplishing countercurrent heating/cooling.
Similarly, as
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shown in United States Patent No. 4,711,298, ceramic block exchangers (similar
to the
graphite block exchangers) are known, but ceramic material also has the
brittle qualities of
graphite so the tubing arrangements are not simple enough for slurries. The
inability of the
existing technology to serve the needs of industries such as the mining
industry is confirmed
by the fact that a need exists, but there are no simple heat exchangers to
serve that need.
SUMMARY OF THE INVENTION
According to a first embodiment of the present invention, a heat exchanger
includes a
heat-exchange section including a plurality of parallel substantially straight
tubes in a close-
spaced geometrical pattern in contact with a solid heat-conductive medium,
including a first
group of tubes and a second group of tubes alternating with the first group of
tubes. The first
and second groups of tubes are in contact with a heat-conductive medium and
are thermally
coupled to one another via the heat-conductive medium. In some embodiments of
the
invention, the first and second groups of tubes are embedded in a heat-
conductive matrix. In
one embodiment of the invention, the exchanger looks like a shell and tube
heat exchanger,
only the fouling shell side is replaced with a solid matrix of heat conductive
material. A fluid,
such as a slurry, to be either heated or cooled is flowed through the first
group of the tubes
(e.g., one half of the tubes) in a first direction. A second fluid is flowed
through the second
group of tubes in the opposite direction of the flow of the first fluid. The
second fluid either
provides or absorbs the heat required. The first and second groups of tubes
are in contact with
a heat-conductive medium and are thermally coupled to one another via the heat-
conductive
medium. In some embodiments of the invention, the first and second groups of
tubes are
embedded in a heat-conductive matrix. In one embodiment of the invention, the
exchanger
looks like a shell and tube heat exchanger, only the fouling shell side is
replaced with a solid
matrix of heat conductive material. A fluid, such as a slurry, to be either
heated or cooled is
flowed through the first group of the tubes (e.g., one half of the tubes) in a
first direction. A
second fluid is flowed through the second group of tubes in the opposite
direction of the flow
of the first fluid. The second fluid either provides or absorbs the heat
required.
The first and second groups of tubes are in contact with a heat-conductive
medium
and are thermally coupled to one another via the heat-conductive medium. In
some
embodiments of the invention, the first and second groups of tubes are
embedded in a heat-
conductive matrix. In one embodiment of the invention, the exchanger looks
like a shell and
tube heat exchanger, only the fouling shell side is replaced with a solid
matrix of heat
conductive material. A fluid, such as a slurry, to be either heated or cooled
is flowed through
the first group of the tubes (e.g., one half of the tubes) in a first
direction. A second fluid is
flowed through the second group of tubes in the opposite direction of the flow
of the first
fluid. The second fluid either provides or absorbs the heat required.
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A second inlet manifold is disposed at a second end of the heat-exchange
section and is fluidly coupled to
second ends of the second group of tubes. The second inlet manifold is fluidly
coupled to a second inlet. A second
outlet manifold is disposed at the second end of the heat-exchange section and
is isolated from the second inlet
manifold. The second outlet manifold is fluidly coupled to second ends of the
first group of tubes. The second outlet
manifold is also fluidly coupled to a second outlet.
At a first end of the structure, a first inlet is fluidly coupled to first
ends of the first group of tubes through a
first inlet manifold. A first outlet is fluidly coupled to first ends of the
second group of tubes through a first outlet
manifold, isolated from the first inlet manifold.
At a second end of the structure, a second inlet is fluidly coupled to second
ends of the second group of
tubes through a second inlet manifold. A second outlet is fluidly coupled to
second ends of the first group of tubes
through a second outlet manifold isolated from the second inlet manifold. In
one illustrative embodiment of the
present invention, the first inlet and outlet manifolds are oriented in line
with one another and the second inlet and
outlet manifolds are oriented in line with one another. In each case, the
tubes for the outermost manifold pass
through the volume of the innermost manifold.
According to a second embodiment of the present invention, a method for
transfer heat from one fluid to
another, includes providing a heat exchanger having a plurality of parallel
tubes in a close-spaced geometrical
pattern in contact with a heat-conductive medium, the plurality of parallel
tubes including a first group of tubes and a
second group of tubes alternating with the first group of tubes, flowing the
first fluid through the first group of tubes;
and flowing the second fluid through the second group of tubes. The method of
the present invention is particularly
advantageous where at least one of the fluids is a slurry.
In a first aspect of the invention there is a heat exchanger including: a heat
exchanger section having a
plurality of parallel tubes running from a first end thereof to a second end
thereof in a close-spaced geometrical
pattern; a solid heat-conductive matrix in contact with the plurality of
parallel tubes; a first manifold coupled to a
first end of the heat-exchange section and fluidly coupled to first ends of a
first group of tubes; a second manifold
coupled to a distal end of the first manifold and isolated from the first
manifold by a first bulkhead disposed at an
acute angle with respect to the first end of the heat-exchange section, the
second manifold fluidly coupled to first
ends of a second group of tubes alternating with the first group of tubes, the
first ends of the second group of tubes
extending through the first manifold and passing through the first bulkhead to
reach the second manifold; a third
manifold disposed at a second end of the heat-exchange section and fluidly
coupled to second ends of the first group
of tubes; and a fourth manifold coupled to a distal end of the third manifold
and isolated from the third manifold by
a second bulkhead disposed at an acute angle with respect to the second end of
the heat-exchange section, the fourth
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manifold fluidly coupled to second ends of the first group of tubes, the
second ends of the first group of tubes
extending through the third manifold and passing through the second bulkhead
to reach the fourth manifold.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. IA is an axial cross-sectional view of an illustrative heat exchange
section of a heat exchanger
according to a typical embodiment of the present invention.
FIG. lB is a radial cross-sectional view of the illustrative heat exchange
section of a heat exchanger of FIG.
IA.
FIG. 2 is a cross-sectional diagram of an illustrative heat exchanger
according to the principles of the
present invention.
FIG. 3 is a diagram of a process employing an illustrative heat exchanger
according to the principles of the
present invention.
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DETAILED DESCRIPTION
Persons of ordinary skill in the art will realize that the following
description of the
present invention is illustrative only and not in any way limiting. Other
embodiments of the
invention will readily suggest themselves to such skilled persons.
Referring now to FIG. 1A and 1B, axial and radial cross-sectional views show
an
illustrative heat exchange section 10 of a heat exchanger according to a
typical embodiment
of the present invention. Heat exchange section 10 may include an outer shell
12 having a
first end 14 and a second end 16. A plurality of tubes 18 run through the heat
exchange
section. The tubes 18 are arranged as two sets of parallel tubes, and are
alternated in a close-
spaced geometric pattern in such an arrangement and spacing in order to ensure
the effective
flow of heat from the fluid in one set of tubes to the fluid in the adjacent
set.
Tubes 18 are held in a heat conducting-medium 20. Heat conductive medium could
be a solid matrix formed, for example, from any of a variety of heat-
conductive materials
which can be drilled or cast, such a metals, ceramics, composites, glasses,
plastics, graphite,
etc. The heat-conductive medium 20 thermally couples adjacent ones of the
tubes to one
another.
According to various embodiments of the present invention, a multiplicity of
tubes
(from 2 to more than several hundred) are arranged in a parallel tube bundle
similar to the
bundle used in a shell-and-tube exchanger. Tube dimensions and geometries
similar to those
employed in shell-and-tube exchangers can be used. Tube sizing and spacing is
selected to
maximize heat transfer between the tubes and the heat-conductive medium. A
solid matrix of
a heat-conductive material such as aluminum, epoxy, ceramic, etc., is disposed
around the
tubes.
In other embodiments of the present invention, the heat exchange section may
be
manufactured by drilling or casting two or more parallel close spaced bores
into a solid
matrix, the matrix formed from a material chosen because it is easy to cast or
drill and
because it transmits heat easily, and then inserting and bonding tubes of a
second material,
chosen because of its chemical (non reactive) properties or because it is
resistant to wear, into
the holes such that a good bond is formed having a low thermal impedance which
transmits
heat from one tube to the other. The bond may be formed by pouring a filler
material into the
gap between the tubes and the holes, or by swaging (expanding) the tubes
outward against the
inner walls of the bores.
Referring now to FIG. 2, a cross-sectional diagram shows a heat exchanger 30
according to the present invention in which the heat exchanger section 10
depicted in FIGS.
1A and iB coupled to a first inlet 32 and to a first outlet 34 through an
inlet manifold 36 and
an outlet manifold 38, respectively, at one end of an illustrative heat
exchanger constructed
according to the principles of the present invention.
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Inlet manifold 36 is separated from outlet manifold 38 by tubesheet 42.
Tubesheet 42
prevents mixing of the two sets of solutions, but allow the fluids or slurries
in each set of
tubes to flow in a single pass from end to end of the exchanger. Each of the
inlet and outlet
manifold 36 and 38 is designed to collect all of the flow from one set of
tubes (or to introduce
such flow into the set of tubes). The tubing diameters and manifold are
designed and arranged
in such a manner as to allow uniform flow of slurries (such as mineral
slurries in a water-
based or corrosive solution) without settling out of solids, and preventing
the mixing of the
flows from the two different sets of tubes, as shown in FIG. 2. In the
illustrative embodiment
shown in FIG. 2, the inlet manifold 36 is bolted to heat exchange section 10
at mating flanges.
Similarly, tubesheet 42 and outlet manifold 38 are bolted to inlet manifold 36
at mating
flanges. This construction facilitates the disassembly of heat exchanger 30
for repair or
maintenance.
As shown in FIG. 2, the first ends of tubes 18a, 18b, and 18c communicate with
inlet
manifold 36 and the first ends of tubes 18d, 18e, and 18f, alternating with
tubes 18a, 18b, and
18c, communicate with outlet manifold 38. At the other end of heat exchange
section 10, the
second ends of tubes 18a, 18b, and 18c communicate with an outlet manifold 44
and the
second ends of tubes 18d, 18e, and 18f communicate with an inlet manifold 46.
Outlet
manifold 44 communicates with outlet 48 and inlet manifold 46 communicates
with inlet 50.
Inlet manifold 46 is separated from outlet manifold 44 by tubesheet 52.
Tubesheet 52 prevents
mixing of the two sets of solutions. Inlet manifold 46, outlet manifold 44,
and tubesheet 52
may be bolted to each other and to the second end of heat exchanger section 10
at flanges to
facilitate the disassembly of heat exchanger 30 for repair or maintenance.
While FIG. 2 shows
heat exchanger 30 in a vertical orientation, persons of ordinary skill in the
art will appreciate
that a horizontal or angled orientation may be employed.
As described above, the ends of heat exchanger 30 are configured in such a
manner
that solution or slurries (fluid) will maintain a simple and uniform flowpath.
In the
embodiment just described, the flow in a first direction will enter through a
header pipe into
an inlet manifold at the first end of the heat exchanger, through the first
set of tubes in the
heat exchange section, through an outlet manifold at the second end of the
heat exchange
section for collecting all the fluid and distributing it to a header pipe in
the side of the
manifold. The flow in a second direction will enter through a header pipe into
an inlet
manifold on the second end of the heat exchanger, through the second set of
tubes in the heat
exchange section, through an outlet manifold at the first end of the heat
exchange section for
collecting all the fluid and distributing it to a header pipe in the side of
the manifold. The
seals between the tubes and the tubesheets 42 and 52 may be made using
compression fittings
or O-ring seals such that the manifold assemblies and tubesheets can be easily
removed for
servicing. Persons of ordinary skill in the art will appreciate that the inlet
and outlet functions
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of one of the sets of manifolds could be reversed according to another
embodiment of the
present invention such that a concurrent flow arrangement instead of a
counterflow
arrangement is realized.
Persons of ordinary skill in the art will appreciate that the ends of the
first and
seconds sets of tubes may be coupled to one another using structures other
than the manifolds
described above. As a non-limiting example, the ends of the tubes may be
merged with one
another using tubing or piping connections.
In a method according to the present invention, heat may be transferred in a
tubular
heat exchanger from the flowing contents of the first set of tubes 18a, 18b,
and 18c to the
flowing contents of the second set of tubes 18d, 18e, and 18f by arranging for
the flow to
occur as a single pass from one end of the exchanger to the other, either co-
current (flowing
contents in both sets of tubes enter at the same end of the exchanger) or
countercurrent in
which a first solution or slurry enters the first end 40 of the heat exchanger
30 and exits at the
second end, while a second solution or slurry enters the second end and exits
at first end 40.
In operation, the slurry to be heated or cooled is run in a first set of the
tubes, and a
similar slurry with a different heating profile is run in a second set of the
tubes, each tube in
the second set adjacent to at least one of the tubes in the first set of
tubes. The present
invention is particularly useful when the two slurries are run in opposite
(countercurrent)
directions, thus heating one slurry while cooling the other. In a typical
system it is possible to
heat a slurry in such a countercurrent configuration from room temperature to
a very high
temperature (e.g., 200 C) against a returning heated slurry which is cooled
from the high
temperature to room temperature. The temperature approach of the two slurries
can be as
close as a few degrees C, so that even though the slurry at the high-
temperature end may be
at, for example, 200 C, only enough heat needs to be added to raise the
slurry a few degrees
C.
The size and number of tubes can vary over a wide range similar to the
variation
which is already practiced in the fabrication of shell and tube exchangers or
tubular
exchangers. Tubing diameter can range from smaller than V2 inches to 3 inches
or more
depending on the type of slurry or process fluid and the cost tradeoffs in
building and
servicing the exchanger. Similarly, number of tubes can vary from two tubes to
a very large
number (similar to some shell and tube exchangers that have more than 1,000
tubes),
depending on the total flowrate of the liquid or slurry to be processed.
The spacing of tubes is more important in the present invention than in a
typical shell
and tube exchanger, and is based on mathematical modeling of heat flow from
tubes in one
set to the alternating (intercalated) tubes in the second set. If the tubes
are too far apart, then
the heat leaving the tube and entering the heat transfer matrix can flow
parallel to the tube
axis and enter either the same tube or the adjacent tube at a non-
perpendicular point. This
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results in "smearing" of the heat transfer effect. In the best geometry, heat
flows directly out
of one tube and into the adjacent tube using the shortest flowpath which is a
straight line
flowpath perpendicular to the tube axes. This requires close tube spacing, but
if the tubes are
too close, then construction of the exchanger is difficult and expensive. The
best matrix is not
necessarily made from the most heat-conductive material, but rather from a
material that
maximizes perpendicular heat flow within the tube spacing constraints. In
practice for most
materials and mediums, the center-to-center spacing of the tubes may be
between about 1/8
inch and about 3/4 inch greater than the tube diameter.
If the heat exchanger is designed properly, most of the heat flows
perpendicular to the
tube axes. This results in the heat exchanger having a very large number,
almost an infinite
number, of theoretical heat exchange stages. With this design it is possible
to get a very close
temperature approach from the liquid or slurry flowing in one set of tubes to
the liquid or
slurry flowing in the other set of tubes. This is one of the distinguishing
features of the
present invention. In a shell and tube exchanger with fluid in the shell, the
number of
theoretical stages is dictated by the effectiveness of the flowpath in the
shell and is usually a
small number. As an extreme example, heat can be extracted as steam in the
shell from a
slurry in the tubes, but in this case there is only one theoretical stage of
transfer regardless of
the length of the exchanger, since the steam in the shell is all at the same
temperature. The
effect of a low number of theoretical stages is that the exchanger must be
much longer to
achieve the same temperature profile as a heat exchanger with a large number
of theoretical
stages. In shell and tube exchanger configurations processing slurries, the
extracted heat must
be sent to a second exchanger where the heat is then transferred to the other
slurry. The
present invention allows the efficient design of an exchanger for the
extraction and
simultaneous transfer of the heat when the flowing fluid is a slurry.
In one embodiment of the present invention, stainless steel tubes with an OD
of 5/8
inches and an ID of V2 inches were placed in a block-centered matrix 7/8
inches on centers,
and a solid aluminum matrix was cast around them. The thickness of the "shell"
matrix
surrounding the outer row of tubes was varied, and it was determined that the
thickness in this
area should be approximately half the tube diameter. Although small-diameter
tubes are very
effective at transferring heat, some slurries (because they possess much
higher viscosities)
need to be processed through much larger tubes. Also for process plants with
very high liquid
or slurry flowrates, larger tubes may be selected because the capital and
maintenance costs of
the exchanger increase as the tubing diameter decreases.
During the development process of the present invention, different heat
exchangers
were formed by casting aluminum, zinc and copper matrices around 5/8" O.D.,
1/2" I.D.
stainless steel tubes on about 7/8" centers. Using water flowing counter
currently against
water, a heat transfer coefficient (from the liquid in one set of tubes to the
liquid in the other)
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of more than 400 BTU per hour per sq. ft. per F was achieved. Shell and tube
exchangers
typically achieve 300 BTU per hour per sq. ft. per F for the transfer of heat
from the liquid in
the tubes to the liquid in the shell.
Referring now to FIG. 3, a diagram shows an illustrative process employing an
illustrative heat exchanger according to the principles of the present
invention. The process
starts at reference numeral 60 using an ore slurry at 50 F and ambient
pressure. Pump 62
raises the pressure of the slurry to 450 PSI. The slurry is then pumped
upwardly through the
heat exchanger 64, in which it receives heat from slurry traveling downwardly
through heat
exchanger 64. At the output of the heat exchanger 64, the temperature of the
slurry is 350 F
at a pressure of 430 PSI. The slurry travels into autoclave 66 having a heater
68 that raises its
temperature 20 F to 370 F where it is processed. The slurry then travels
back down through
the heat exchanger 64 in which it transfers heat to the counterflowing slurry
moving upwardly
through heat exchanger 64. The slurry exits the bottom of heat exchanger 64 at
a temperature
of 70 F and a pressure of 410 PSI. The slurry then passes through pressure-
reducing valve 70
and, at reference numeral 72 passes to downstream processes at a temperature
of 70 F at
ambient pressure.
The present invention satisfies a long felt and unsatisfied need for equipment
that can
effectively and efficiently transfer heat from one mineral ore slurry to
another, and thus
represents a major advance over existing prior-art heat exchangers that have
not met this
need.
While the present invention is disclosed in the context of heat exchange in
mineral
slurries in the mining industry, persons of ordinary skill in the art will
recognize that it has
much broader uses than mineral slurries, such as in various processes employed
in the
pharmaceutical and chemical industries that currently use other types of
exchangers. The heat
exchangers presently used in these industries have complicated geometries,
sharp corners, and
passage configurations that are difficult to clean.
A simple configuration in accordance with the present invention in which all
fluids
moving in either direction move through round, straight, non-fouling tubes
which are easily
accessible, and easily cleaned solves the problems of the prior art. The
present invention
provides a highly efficient, hitherto unavailable heat exchanger that
significantly reduces
maintenance and operating costs.
While embodiments and applications of this invention have been shown and
described, it would be apparent to those skilled in the art that many more
modifications than
mentioned above are possible without departing from the inventive concepts
herein. The
invention, therefore, is not to be restricted except in the spirit of the
appended claims.
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